The invention relates to raster correction circuits of a video display.
In a cathode ray tube (CRT) of a video display, a raster is formed by deflecting an electron beam across a phosphor screen. Each electron beam is deflected in a horizontal direction by a magnetic field produced by in a horizontal deflection coil by a horizontal-rate sawtooth current. Likewise, the electron beam is simultaneously deflected in a vertical direction by a magnetic field produced by a vertical deflection coil by a vertical-rate sawtooth current. The result is a negatively-sloped, or xe2x80x9cdownhillxe2x80x9d, scan line as the electron beam is deflected from left to right to form the CRT""s raster. In a typical cathode ray tube used in a color television receiver and having, for example, a screen width of approximately 723 mm and a screen height of approximately 538 mm, a horizontal scan line may drop a distance of approximately 2.4 mm from a perfectly horizontal position in one field. This downhill scan effect introduces both orthogonality and parallelogram errors into the raster.
In a perfectly rectangular raster, horizontal and vertical center lines are orthogonal, or perpendicular, to one another. Downhill scanning does not produce a perfectly rectangular raster and hence results in a non-orthogonal relationship between the horizontal and vertical center lines of the raster. Orthogonality error is a quantitative measure, expressed in units of radians or degrees, of the extent to which the horizontal and vertical center lines of a raster depart from orthogonality. The orthogonality error may be magnified at the left and right edges of the raster because the deflection sensitivity increases near the edges of the raster. As a result, the edges of the raster may tilt such that the raster has a generally parallelogram shape. errors in a raster can be obtained by providing a horizontal-rate modulation of a vertical deflection current for substantially offsetting the downhill scan effect caused by vertical deflection of the electron beam. In one of the circuits shown
Elimination of both orthogonality and parallelogram errors in a raster can be obtained by providing a horizontal-rate modulation of a vertical deflection current for substantially offsetting the downhill scan effect caused by vertical deflection of the electron beam. A winding of a horizontal flyback transformer can be used to apply a horizontal retrace pulse voltage to a primary winding of a transformer. A secondary winding of the transformer can be coupled to a vertical deflection winding for providing a small horizontal rate sawtooth current to be superimposed on a vertical deflection current.
Coupling back of the vertical current to the horizontal deflection circuit is reduced by the relatively large leakage of the transformer. Nevertheless, the residual vertical rate current, during vertical retrace, can still produce a disturbance at the top of the screen, immediately after vertical retrace. It may be desirable to further reduce the coupling back of the vertical current to the horizontal deflection circuit.
In carrying out an inventive feature, an R-C filter is interposed in a current path between the transformers. The R-C filter attenuates the coupled back vertical deflection current. Thereby, the addition of the R-C coupling filter prevents the vertical deflection current from affecting the horizontal deflection circuit.
A video display deflection apparatus, embodying an inventive feature, includes a first deflection circuit for generating a first deflection current at a first deflection frequency in a first deflection winding to vary a position of an electron beam in a first direction. A second deflection circuit is used for generating a second deflection current in a second deflection winding at a second deflection frequency to vary the position of the electron beam in a second direction. A filter couples the second deflection circuit to the first deflection winding to generate a corrective current in a current path formed by the first deflection winding at a frequency related to the second deflection frequency for providing raster error correction. The filter significantly attenuates parasitic signal coupling in an opposite direction, from the first deflection circuit to the second deflection circuit.